Multiple-Input-Multiple-Output (MIMO) may increase the spectrum efficiency of the wireless communications system exponentially through utilizing space resources and therefore has become one of important techniques of cellular communications. However, in order to obtain the spectrum gain, a transmitter has to know Channel Direction Information (CDI), so as to perform precoding calculation and other MIMO signal processing. The CDI and Channel Quality Information (CQI) form complete Channel State Information (CSI). For a MIMO system, it is a prerequisite for close-loop MIMO transmission that the transmitter obtains accurate CDI, which is also a key factor affecting system performance.
Long Term Evolution (LTE) system corresponding to the Evolved Universal Terrestrial Radio Access (E-UTRA) protocol provided by the 3rd Generation Partnership Project (3GPP) has different CDI obtaining manners with respect to different duplexing modes. The duplexing modes of the LTE include: Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD).
In a TDD system, uplink channel and downlink channel have a property of reciprocity. Therefore, a TDD base station may obtain the equivalent CDI of the downlink channel through performing channel estimation to the uplink channel. For assisting the channel estimation, the terminal transmits omnidirectional Sounding Reference Signal (SRS) which is generated adopting a particular pseudo-random sequence, e.g. Zadoff-Chu (ZC) sequence. Both the terminal and the base station know the adopted sequence. The biggest defect of obtaining the CDI based on transmitting the SRS and the channel estimation in the TDD system is a pilot contamination problem. In the LTE system, the SRS sequences assigned to different terminals of the same cell are orthogonal. Therefore, the base station may perform channel estimation without any interference according to the SRS sequences of different terminals to obtain the CDI of their uplink channels. However, in the LTE system, the SRS sequences assigned to the terminals in different cells are non-orthogonal, and even a plurality of terminals may use the same SRS sequence, i.e., the so-called SRS collision. In the case of SRS collision, the base station also receives uplink SRS signals from terminals in other cells while estimating the uplink channel CDI of the terminal of the present cell. Therefore, the CDI of the channel of the present cell estimated by the base station also involves the CDI of the channels between the terminals of the other cell and the base station, which is referred to as pilot contamination. The pilot contamination has serious impacts to both uplink and downlink data transmissions: 1) when the base station transmits data with directional precoding via a downlink channel to an expected terminal, the directional data is also transmitted to the terminals of the adjacent cell on the co-channel, which results in serious inter-cell interference; 2) if the base station performs directional post-processing to receive data from the expected terminal via an uplink channel, enhancement processing is also performed to the data of the terminal of the adjacent cell on the co-channel, thus the interference on the co-channel is enlarged. Due to the above reasons, the pilot contamination seriously restricts the system capacity. Especially when the number of antennas increases, there will be a bottle neck for the promotion of the system performance.
Large-scale antenna array system (large-scale MIMO or Massive MIMO) is a main candidate technique for the 5th generation cellular communications standard. The large-scale antenna makes it possible to use a high signal processing freedom to greatly reduce the interference between terminals and interference between cells. It has low computation complexity and is able to effectively improve quality of communication links. In addition, the large-scale antenna is able to effectively decrease power consumption of a single antenna unit and increase the energy efficiency of the whole system. Existing experiments have sufficiently proved the feasibility of configuring dozens or even hundreds of antennas for one base station. One implementation on the millimeter wave band is as follows: through configuring the large-scale antenna array for the base station, when the distance between antennas is very small, an extremely narrow transmission beam is formed utilizing phase difference between antennas to serve multiple terminals. At the same time, the terminal may also be configured with multiple antennas to produce different gains for different directions-of-arrival and select a beam with a larger gain to realize data receiving. If each transmission beam of the base station serves one terminal, the interference between terminals are decreased greatly. If adjacent base stations serve their respective terminals using transmission beams in different directions, the interference between cells is decreased greatly. Theoretical result shows that, in a large-scale antenna system, if the transmitter knows the accurate CDI of the channel of the terminal, the available Signal-to-Noise Ratio (SNR) of the uplink and downlink channels increases with the increase of the number of the antennas. In the case of dozens of or even hundreds of transmission antennas, the system capacity is dramatically increased accordingly. However, if there is pilot contamination, the actual capacity of the large-scale antenna system will decrease sharply, even if the transmission power of the base station is relatively low, the whole system is interference limited. The pilot contamination has a fatal impact to the large-scale antenna system. Therefore, it is significant to design a new CDI obtaining manner to overcome the pilot contamination problem in the large-scale antenna system.
In the FDD system, the uplink and downlink channels are asymmetric since they are on different frequency bands. The base station cannot obtain the downlink channel CDI through estimating the uplink channel. In this situation, the terminal has to feed back the CDI and CQI of the downlink channel to the base station by occupying some uplink channel resources. One method is explicit feedback. The terminal quantizes the CDI of the downlink channel with a fixed codebook and performs a multi-level quantization to the CQI, and reports the quantized result to the base station via the uplink channel. Another method is implicit feedback. The terminal selects an expected precoding from several fixed precoding according to the CDI of the downlink channel, and reports the selected result and the CQI corresponding to the selected CDI to the base station via the uplink channel. In order to realize the above, the base station has to perform precoding to a reference signal with different CDI. The terminal measures the reference signal, so as to obtain the signal power of the corresponding CDI and determine the CQI. Whichever method is adopted, the FDD system has to bear two kinds of necessary overheads in order to enable the base station to obtain accurate CSI of the downlink channel: reference signal overhead and feedback overhead. At the same time, the two kinds of overheads must increase with the number of antennas. Higher feedback quantization accuracy means higher overhead. This means that the manner for obtaining the CDI based on the feedback in the FDD system faces a challenge on how to reduce overhead when the large-scale antenna system is applied.
Another important problem is how to perform precoding to the downlink data channel in the presence of the CDI measurement error and the feedback latency. If the CDI feedback accuracy is low and the terminal moves with a high speed, the downlink data of the base station may deviate from the optimum channel direction and thus result in decrease of system performance.
In view of the above, in the design of the 5G communications system, it is an urgent problem to be solved as to the obtaining of the CDI in the large-scale antenna array system. Through designing a rapid and effective CDI obtaining method, reference signal and signaling overheads of the system may be decreased effectively. At the same time, it avoids the possibility that the base station uses incorrect CDI, ensures the spectrum gain brought out by the large-scale antenna, so as to increase the system capacity of the cell.